The present invention relates to the hot forming or forging of metal workpieces and, more particularly, to an improved method of precisely controlling the finished thickness of the forged metal workpieces. The term metal, as used herein, includes both elemental metals and alloys unless indicated otherwise.
Numerous methods for the solid state forming of metallic workpieces or blanks into selected shapes include forging and rolling. Press forging and trimming are two widely used techniques in which the metal is worked at an elevated temperature such as for the formation of gas turbine engine blade airfoils. In a typical forging operation, an unformed workpiece is pre-heated to forging temperature and then shaped with a hammer or ram of a forge press. The unformed workpiece is typically a pre-form having an approximate shape to that of the formed workpiece. In a typical trimming operation, the formed workpiece is trimmed while still hot from the forging process and excess metal and/or flash formed during the forging process is trimmed using trimming dies and a hammer or ram of a trim press. The unformed workpiece is typically a pre-form having an approximate shape to that of the formed workpiece.
The hot forming or forging process requires a heated workpiece at a high temperature, typically above 1,700° F. The forge dies, though often heated, are at a much lower temperature, typically less than 500° F. or even at room temperature. The large temperature differential and the high thermal diffusivity of the metals being forged causes a rapid heat transfer. The temperature of the workpiece in contact with the die drops in temperature at a rate of 100° F. per second or more. The thinner the workpiece the larger the relative effect of this temperature drop. In the absence of any other reheating methods, the temperature of the workpiece falls in a transient way until the ram of forge press impacts the upper die against the workpiece. Afterwards, the workpiece temperature continues to fall until the workpiece is removed from contact with the metal die or until it reaches the same temperature as the die. The trimming process is similar in that the formed workpiece is at a substantially higher temperature than the trim dies.
The physical properties of the workpiece material at time of impact of the forge press on the workpiece are primarily a function of the temperature at time of impact. These physical properties contribute to the results of the forging process in terms of extent of deformation achieved with a specific forge force as well as the flow of material caused by the forge forces. In addition there is heat generated in the material during deformation caused by the plastic deformation which also affects the results of the forging process. A similar situation exists for the trimming process as regards the deformation in terms of change in shape of the workpiece. This deformation is relatively minor compared to that during forging. On the other hand, the trim size itself and the orientation of features of the workpiece relative to each other can be significantly affected.
The conditions of the workpiece at the exact instant of impact by the ram are determined by the transient temperature distribution through the workpiece which, in turn, is determined by the heat transfer from the workpiece to the die. The heat transfer primarily depends on two parameters: (1) the heat transfer coefficient or resistance to the heat transfer from the workpiece to the die and (2) the time of contact with the colder die during which the heat transfer takes place.
Variations in these two parameters during the forging and trimming processes affect repeatability of the processes and hence the consistency of the parts that are forged and trimmed. It is very desirable to have a high degree of repeatability in forging and trimming processes and forged and trimmed parts that are more consistent.
U.S. Pat. No. 6,223,573 B1, issued to applicants, is directed to methods and apparatus for operating a press that includes an upper die that controllably impacts a lower die to shape a workpiece that is placed on the lower die. The upper die is controllably monitored either to impact the lower die after a workpiece contacts the lower die for a predetermined fixed period of time, or to impact the lower die some predetermined fixed period of time after sensing a predetermined fixed temperature of a predetermined location of the workpieces. This controlled monitoring produces improved repeatability of workpiece. However, by providing fixed periods of time for each controllably monitored process, the precision of parts produced is improved, but limited. This limitation does not employ a precise relationship between a finished thickness of a workpiece, the temperature of the workpiece, and the period of time the workpiece is in contact with the lower die period to impact with the upper die. With this precise relationship, which does not use fixed time periods to account for changing conditions, workpieces produced by the present invention, which is discussed in further detail below, achieves significant precision improvement that makes workpiece thickness control up to about 0.0015 inches possible.
The variation in the heat transfer coefficient and the time of contact causes substantial variations between parts in the temperature profile in the workpiece and thereby causes variation in the shape, form and thickness of the product. This variation is significant because of the precision required in gas turbine engines and, in particular, aircraft gas turbine engine airfoils. In the case of airfoils, which are of thin construction, typically tapering to approximately 0.050 inches at the edge of the airfoil, the rate of temperature drop upon contact with the lower die is significantly larger than the rest of the part. As such, the flow stress in the airfoil increases rapidly as the temperature of the airfoil decreases. As used herein, the term “thin parts” refers to parts having a significant amount of edge thickness of about 0.10 inches or less, which edge thickness being critical to the successful operation of the part. It has been found that for thin parts, thickness control has been difficult to achieve more precisely than about 0.008 inches. That is, a range from about 0.004 inches above a nominal thickness of the part to about 0.004 inches below the nominal thickness of the part. In addition to the range of part thickness, there is also a range associated with the orientation of the airfoil, or in other words, the twist of the airfoil.
Various corrective actions are currently used in forge shops to reduce these variations. Adjustments of other press and forming parameters, benching and changing the shape of the dies, subsequent cold working and hot working, chemical metal removal are all used to reduce part-to-part variation to meet tolerance requirements. These corrective operations increase the cost of production and inventory, due to the number of additional tools required, and also increase the cycle time for making the part.
Any variation in the temperature of the workpiece at the instant of impact during operation of trim and forge presses affects the stress and deformation of the workpiece which then causes a variation in the orientation and thickness of portions of the part. In the case of an airfoil of a gas turbine engine blade, in addition to the variation in the shape and thickness of the part, workpiece temperature variation also causes variations in the orientation of the airfoil with respect to the dovetail and platform. In the trim process it also causes variations in the chord length of the airfoil. These variations cause difficulty in meeting the tolerance requirements of the component. Subsequent operations to manually bench or deform the part to conform to the orientation required and to grind the chord length add significant cost to the part in time required to produce the part and in the additional holding fixtures and tools required by these additional operations. For the high degree of precision required in aviation airfoils, this variation result in substantial cost increases. Parts that deviate to a further extent from the nominal dimensions and/or profiles require still further adjustments or other press and forming parameters, or are scrapped.
Another factor that affects repeatability or part-to-part variation is the additional variability due to operators working at different speeds and variations during the shift of the same operator. These differences cause both the time of contact and the heat transfer coefficient to vary with consequent variation in the part geometry. There is a need to reduce part-to-part variation in the forging and trimming processes using presses and improve consistency of hot formed parts made with forge and trim presses.